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Magnetic holes are coherent structures associated with a strong depression in the magnetic field amplitude. Such structures are ubiquitous in space plasmas and are observed in the solar wind, in planetary bow shocks and magnetosheaths, in the Earth's magnetotail and around comets. Magnetic holes may have very different sizes and properties. The largest ones have a size of hundreds of ion gyroradii while the smallest ones are sub-ion scale structures of the order of a few electron gyroradii. The drop in magnetic field amplitude associated with magnetic holes is often sustained by an increase in plasma density and enhanced ion and electron temperature anisotropies, with temperatures that are typically higher in the plane perpendicular to the local magnetic field. These properties seem to suggest that the generation of magnetic holes may result from the nonlinear evolution of mirror modes whose growth is fed by perpendicular temperature anisotropies and that are characterized by anticorrelated magnetic field and density perturbations. Some observational and numerical studies seem to support the idea of a scenario in which magnetic holes are generated by the mirror instability but in many cases this picture is not consistent with observations, especially in the case of sub-ion scale magnetic holes for which a number of possible generation mechanisms have been considered. Hence, the origin of magnetic holes is still controversial and under debate. Plasma turbulence is also known as a driver for the generation of coherent structures and may play a key role in the formation of magnetic holes, especially in the solar wind and in the Earth's magnetosheath that are in a turbulent state. Indeed, numerical simulations of plasma turbulence show that sub-ion scale magnetic holes can develop self-consistently out of small scale magnetic fluctuations that locally reduce the magnetic field amplitude and trap hot electrons. However, it is still unclear how such small scale fluctuations can emerge in a turbulent plasma where energy is typically injected at large scales. In this work, we study the formation of sub-ion scale magnetic holes by means of fully kinetic particle-in-cell simulations of plasma turbulence. We show that by injecting energy at scales relatively large with respect to ion scales, the turbulence naturally tends to generate sub-ion scale electron velocity shear layers associated with elongated magnetic field grooves. These elongated magnetic dips then become unstable and break up into sub-ion scale magnetic holes characterized by an intense azimuthal electron current and a strong perpendicular electron temperature anisotropy. We show that the properties of magnetic holes generated by this mechanism are consistent with satellite observations. Our results may provide a possible explanation of how magnetic holes develop in a realistic turbulent environment.

Mercury is the only telluric planet of the solar system, other than Earth, with an intrinsic magnetic field. Thus, the Hermean surface is shielded from the impinging solar wind via an “Earth-like” magnetosphere. This magnetic cavity is however twenty times smaller than its counterpart at the Earth and Mercury occupies a large portion of this cavity. The relatively small extension of the Hermean magnetosphere enables global numerical modeling of this system. Past modeling efforts have addressed both fluid and ion kinetic plasma physics, therefore supporting past in-situ plasma observations at Mercury by the NASA MESSENGER mission. Inspired by the wealth of observations provided by MESSENGER, the ongoing ESA/JAXA BepiColombo mission started probing the plasma environment around Mercury with a devoted instrumental suite targeting for the first time the electron kinetic physics. To interpret ongoing – and prepare future – BepiColombo observations of Mercury’s magnetosphere, global simulations of the Hermean environment including electron kinetic physics are crucial. The goal of this work is to self-consistently study the global electron dynamics in the Hermean magnetosphere. A particular focus is given to (i) electron acceleration and heating as a consequence of magnetic reconnection, (ii) electron circulation in the Hermean dipolar magnetic field and, finally, (iii) precipitation of electrons on Mercury’s surface. The investigation of such processes requires a dynamical model encompassing a wide range of temporal and spatial scales going from the large planetary (fluid) scales down to the small electron (kinetic) scales. To this purpose, we present three-dimensional, fully-kinetic simulations of the interaction between the solar wind plasma and Mercury’s dipole magnetic field. This model self-consistently includes plasma processes from the large planetary scale down to the electron scale. Such simulations are performed with the implicit, full-PIC code iPIC3D. From the analysis of our global simulations, we find the following results. (i) Magnetic reconnection in the tail accelerates and heats electrons up to tens of keV in the case of southward interplanetary magnetic field (IMF). These electrons are ejected from the neutral line in the tail planet-ward in a substorm fashion. (ii) Such substorm electrons drift dawn-ward from the nightside around the planet, forming a quasi-trapped population. Noticeably, most of these electrons fall on the planet surface before completing a full longitudinal orbit. We also show comparisons between our simulations and in-situ observations by Mariner10 and BepiColombo space missions. We argue that these electrons have been observed in-situ by the PLS instrument onboard Mariner10 during its first Mercury flyby around and after closest approach. A similar electron population, although fainter, have been observed by the MPPE/MEA instrument onboard BepiColombo in the nightside magnetosphere. (iii) Eventually, electrons fall on Mercury’s surface and interact with it. We show maps of electron precipitation on Mercury’s surface and discuss them in comparison with observations of Calcium X-ray fluorescence emission from the surface by the XRS instrument onboard MESSENGER. This paves the way to the understanding of future X-ray observations of Mercury’s surface by the MIXS instrument onboard BepiColombo, as a signature of plasma-surface interaction at Mercury.

Mutual impedance experiments are active electric instruments that provide in situ diagnostic in space plasmas, such as the plasma density and electron temperature. The instrumental technique is based on the coupling between electric antennas embedded in the plasma, and characterizes the local properties of the plasma dielectric. Different versions of mutual impedance instruments are present onboard past and future planetary missions, such as Rosetta, BepiColombo, JUICE, and Comet Interceptor. Recently, the interest of the scientific community is shifting from large satellite platforms with single-point measurements concepts to small satellite platforms, to enable multipoint measurements for the spatial mappings of planetary outer environments. Therefore, instruments previously designed for large platforms are now miniaturized and adapted to small satellites. In this context, instrumental efforts are devoted to adapting mutual impedance experiments to small satellites, such as in the case of the CIRCUS CubeSat or the SPEED SmallSat missions projects. Current state-of-the-art quantitative instrumental models of mutual impedance experiments are based on the assumption of an unmagnetized plasma. However, for planetary environments within which the magnetic field is not negligible, such as intrinsic planetary magnetospheres (e.g. Mercury, Ganymede) significant modifications of mutual impedance measurements are expected. The goal of this work is twofold: (i) support the preparation of mutual impedance instruments for small satellites and (ii) extend current mutual impedance instrumental models to take into account the effects of the magnetic field on the plasma diagnostic. This investigation is performed by combining two complementary approaches. First, numerical simulations are used to quantify the impact of the plasma magnetization on the mutual impedance measurements and, therefore, improve its diagnostic. In particular, we have developed and validated a new instrumental model, based on the numerical calculation of the electric potential emitted by an electric antenna in a magnetized, homogeneous, collisionless, Maxwellian plasma. This new instrumental model is used to compute synthetic mutual impedance spectra and assess the impact of electron magnetization on the instrumental response. Using this new model, we provide diagnostics for the plasma density, electron temperature, and magnetic field amplitude. Second, laboratory experiments are used to test and validate our numerical model. We use the controlled plasma environment of the PEPSO plasma chamber at LPC2E laboratory in Orléans. This plasma chamber offers the possibility to test the performances of space plasma instruments and CubeSats in realistic planetary ionospheric conditions. A model of CubeSat is present inside the plasma chamber, and is equipped with a set of electric antennas that are used to perform mutual impedance measurements in the same configuration as a CubeSat. The measurements obtained by this setup are compared with the instrumental model, to validate the plasma diagnostic of the instrument prototype on a CubeSat.

We derive the coarse-graining (CG) equations of incompressible Hall Magnetohydrodynamics (HMHD) turbulence to investigate the local (in space) energy cascade rate as a function of the filtering scale. First, the CG equations are space averaged to obtain the analytical expression of the mean cascade rate. Its application to 3 dimensional (3D) simulations of (weakly compressible) HMHD shows a cascade rate consistent with the value of the mean dissipation rate in the simulations and with the classical estimates based on the "third-order" law. The strength of the CG approach is further revealed when considering the local-in-space energy cascade rate which is shown theoretically and numerically to match dissipative processes at a given position x, when both quantities are locally averaged over a small neighboring region (quasi-locality). This result supports the claim that the (quasi-)local cascade rate can provide a reliable estimate of the (quasi-)local energy dissipation, regardless of the nature of the dissipation processes involved.The new model is further applied to magnetic reconnection sites observed in Hybrid-Vlasov (HVM) simulations and in spacecraft data. The results show the robustness of the new model over standard tools used to highlight energy dissipation processes and particles energization at the X points.

The magnetopause boundary seems to escape the general classification of discontinuities since it mixes characteristics of shocks (magnetic field magnitude increase) and those typical for the rotational discontinuities (magnetic rotation), whereas it is very often described as a tangential discontinuity. As, the main issue is the amount of matter/momentum/energy from the solar wind and entering into the magnetosphere, the solution cannot be simply achieved by assuming the discontinuity as strictly tangential, everywhere and at all times. Here we propose to study the magnetopause boundary as a "quasi-tangential" discontinuity, with the normal magnetic component Bn small but not null since even small departures from the standard hypothesis of a zero Bn can lead to noticeable changes in the global properties. In that aim, we look into the MMS database for a large number of magnetopause crossings. For each case we will determine what are the most important features (non-planarity, non-stationarity, Hall effect, pressure anisotropy and agyrotropy) that allow the discontinuity to escape the general classification, i.e. to noticeably change the form of the conservation laws on which the theory of discontinuities is based for non-strictly tangential discontinuities. We put a special emphasis on the refined methods that can be used for determining the spatial gradients from four spacecraft data and on the accuracy that can be attained by these methods.

Solar wind (SW) in situ observations of plasma turbulence show that the turbulent magnetic field spectrum follows a Kolmogorov-like scaling ∼k-5/3 at large MHD scales and steepens at ion scales where a different power law develops with a scaling exponent varying between -2 and -4, depending on SW conditions. Recent satellite measurements revealed the presence of a second spectral break around electron scales where the magnetic field spectrum shows an exponential falloff described by the so called exp model ∼k-8/3exp(-ρek), where ρe is the electron gyroradius. This model was tested on a large number of magnetic spectra at various distances from the Sun (from 0.3 to 1 AU) and appears to be a solid feature of turbulent magnetic field fluctuations at kinetic scales [1]. Using a fully kinetic energy conserving particle-in-cell (PIC) simulation of freely decaying plasma turbulence we study the spectral properties of the turbulent cascade at kinetic scales. Consistently with satellite observations, we find that the magnetic field spectrum follows the k-αexp(-λk) law at sub-ion scales, with an exponential range developing around kρe≈1. The same exponential falloff is observed also in the electron velocity spectrum but not in the ion velocity spectrum that drops like a power law without reaching electron scales. We investigate the development of these spectral features by analyzing the high-pass filtered electromagnetic work J·E and pressure-strain interaction -P:∇u of both the ions and the electrons. Our analysis shows that the magnetic field dynamics at kinetic scales is mainly driven by the electrons that are responsible for the formation of the exponential range. In particular, we see that at fully developed turbulence the magnetic field energy is dissipated by a two-stage mechanism lead by the electrons that first subtract energy from the magnetic field and then convert it into internal energy at electron scales through the pressure-strain interaction, that accounts for the electron heating [2]. References[1] Alexandrova, O., Jagarlamudi, V. K., Hellinger, P., Maksimovic, M., Shprits, Y., & Mangeney, A. (2021). Spectrum of kinetic plasma turbulence at 0.3–0.9 astronomical units from the Sun. Physical Review E, 103(6), 063202.[2] Arrò, G., Califano, F., & Lapenta, G. (2021). Spectral properties and energy cascade at kinetic scales in collisionless plasma turbulence. arXiv preprint arXiv:2112.12753.

Mercury is the only telluric planet of the solar system, other than Earth, with an intrinsic magnetic field. Thus, the Hermean surface is shielded from the impinging solar wind via the presence of an “Earth-like” magnetosphere. However, this cavity is twenty times smaller than its alike at the Earth. The relatively small extension of the Hermean magnetosphere enables us to model it using global full-kinetic simulation with the aid of modern supercomputers. Such modeling is crucial to interpret, and prepare, the future observations of the ongoing joint ESA-JAXA mission BepiColombo.The model used in this work is based on three-dimensional, implicit full-PIC simulations of the interaction between the solar wind and Mercury’s magnetosphere (i.e. at 0.3-0.47 AU). This model includes self-consistently the ion and electron physics down to kinetic electron scales. On top of that, we show comparisons between in-situ observations by Mariner-X and BepiColombo space missions. This comparison allows us (i) to validate our model and (ii) to gain insights into the electron dynamics in the Hermean environment, thought to be governed by kinetic-scale processes.First, we validate our model through a qualitative comparison between three-dimensional outcomes of our global simulations and the ones of reduced fluid/hybrid simulations (in the context of the SHOTS collaboration). Moreover, comparison with in-situ Mariner-X observations during its first Mercury flyby complete the validation of our model. Second, we study the global dynamics of electrons showing regions where strongest particle acceleration/energization occurs, giving quantitative estimate of electron temperature anisotropy in the Hermean environment. Such results are used to interpret past, and plan future, BepiColombo in-situ observations.

International audience; When spacecraft (s/c) missions probe plasma structures (PSs) the relative location of the s/c with respect to the PS is unknown. This information is, however, needed to measure the geometrical features of the PS (orientation and thickness) and to understand the physical processes underlying the PS dynamics. Methods to determine the s/c location exist, but they need strong assumptions to be satisfied (stationarity and special spatial dependencies). The number of cases for which these assumptions are likely to be valid for the entire PS seems to be limited, and even weak departures from these hypotheses may affect the results. For a quasi‐1‐D geometry in particular, the determination of the velocity component along the two quasi‐invariant directions is very inaccurate and the assumption of strict stationarity may lead these quantities to diverge. In this paper we present new methods to compute the s/c trajectory through a PS, without a priori assumption on its spatial geometry, and able to work even in the presence of weak nonstationarities. The methods are tested both on artificial and real data, the latter provided by the Magnetospheric Multiscale mission probing the Earth's magnetopause. The 1‐D and 2‐D trajectories of the Magnetospheric Multiscale are found that can be used as an initial step for future reconstruction studies. Advanced minimization procedures to optimize the results are discussed.

This letter exploits the radial alignment between the Parker Solar Probe and BepiColombo in late 2022 February, when both spacecraft were within Mercury’s orbit. This allows the study of the turbulent evolution, namely, the change in spectral and intermittency properties, of the same plasma parcel during its expansion from 0.11 to 0.33 au, a still unexplored region. The observational analysis of the solar wind turbulent features at the two different evolution stages is complemented by a theoretical description based on the turbulence transport model equations for nearly incompressible magnetohydrodynamics. The results provide strong evidence that the solar wind turbulence already undergoes significant evolution at distances less than 0.3 au from the Sun, which can be satisfactorily explained as due to evolving slab fluctuations. This work represents a step forward in understanding the processes that control the transition from weak to strong turbulence in the solar wind and in properly modeling the heliosphere.

International audience

Density inhomogeneities are ubiquitous in space and astrophysical plasmas, in particular at contact boundaries between different media. They often correspond to regions that exhibits strong dynamics on a wide range of spatial and temporal scales. Indeed, density inhomogeneities are a source of free energy that can drive various plasma instabilities such as, for instance, the lower-hybrid-drift instability which in turn transfers energy to the particles through wave-particle interactions and eventually heats the plasma. Here, we address the role of this instability in the Hermean plasma environment were kinetic processes of this fashion are expected to be crucial in the plasma dynamics and have so far eluded the measurements of past missions (Mariner-X and MESSENGER) to Mercury. The goal of our work is to quantify the efficiency of the lower-hybrid-drift instability to accelerate and/or heat electrons parallel to the ambient magnetic field.To reach this goal, we combine two complementary methods: full-kinetic and quasilinear models.We report self-consistent evidence of electron acceleration driven by the development of the lower-hybrid-drift instability using 3D-3V full-kinetic numerical simulations. The efficiency of the observed acceleration cannot be explained by standard quasilinear theory. For this reason, we develop an extended quasilinear model able to quantitatively predict the interaction between lower-hybrid fluctuations and electrons on long time scales, now in agreement with full-kinetic simulations results. Finally, we apply this new, extended quasilinear model to a specific inhomogeneous space plasma boundary: the magnetopause of Mercury, and we discuss our quantitative predictions of electron acceleration in support to future BepiColombo observations.

La ricchezza di un discorso filosofico non può essere misurata attraverso gli stessi criteri utilizzati in ambito scientifico, dove il raggiungimento di risposte e risultati oggettivi è fondamentale perché si possa parlare di progresso e successi. In filosofia, al contrario, a destare l’interesse nei confronti di un autore non sono tanto le risposte determinate alle quali egli perviene nel corso della sua ricerca, bensì le domande che è in grado di porre. È la capacità del domandare, di mettere in questione e in crisi sempre e nuovamente l’intero ambito dell’esistenza, ciò che in qualche modo misura la grandezza di un filosofo. Partendo da siffatta premessa, possiamo comprendere perché Peter Sloterdijk abbia riscosso tale risonanza negli ambienti filosofici, ma anche più in generale nei dibattiti culturali e politici, degli ultimi anni. Risonanza caratterizzata, bene sottolinearlo, tanto da apprezzamenti, quanto da atteggiamenti di rifiuto e opposizione.

As part of on-going efforts in advancing air-breathing electric propulsion, the HT5k Hall thruster was characterized in six operating conditions, ranging from 5 mg s−1 to 7 mg s−1 of 0.56N2 + 0.44O2 mass flow rate and 225 V to 300 V of discharge voltage. The cathode was operated with N2 at mass flow rates between 0.5 mg s−1 and 0.7 mg s−1. This paper presents a 0D-hybrid model for atmospheric propellant fed Hall thruster discharges. Verified performance ranged between 30 mN to 120 mN in thrust, 1.2 kW to 5.2 kW in discharge power, and 8% to 18% in anodic efficiency. Calibrated model comparison against experimental data resulted in a mean absolute error of 3.7% in thrust and 7.6% in discharge power.

Magnetic reconnexion and Kelvin-Helmholtz (KH) instability are usually recognized as the two main mixing processes along magnetopauses. However, a recent work [Dargent et al., 2019] showed that in Mercury’s conditions, another instability can grow faster than the KH instability along the magnetopause. This instability seems to rely on gradients of density and/or magnetic field and develops large-scales finger-like structures that prevents the growth of the KH vortices. In this work, I will characterize this instability and try to identify it. In particular, I will look at the dependance of the growth rate of this instability to the different parameters of the plasma and compare it to the growth rate of the Kelvin-Helmholtz instability.

Turbulence in collisionless magnetized plasmas is a complex multi-scale process involving many decades of scales ranging from large magnetohydrodynamic (MHD) scales down to small ion and electron kinetic scales, associated with different physical regimes. It is well know that the MHD turbulent cascade is driven by the nonlinear interaction of low-frequency Alfvén waves but, on the other hand, the properties of plasma turbulence at sub-ion scales are not yet fully understood. In addition to a great variety of relatively high frequency modes such as kinetic Alfvén waves and whistler waves, magnetic reconnection has been suggested to be a key element in the development of kinetic scale turbulence because it allows for energy to be transferred from large scales directly into sub-ion scales through currents sheets disruption. In this context, an unusual reconnection mechanism driven exclusively by the electrons (with ions being demagnetized), called "electron-only reconnection", has been recently observed for the first time in the Earth’s magnetosheath and its role in plasma turbulence is still a matter of great debate. Using 2D-3V hybrid Vlasov-Maxwell (HVM) simulations of freely decaying plasma turbulence, we investigate and compare the properties of the turbulence associated with standard ion-coupled reconnection and of the turbulence associated with electron-only reconnection [Califano et al., 2018]. By analyzing the structure functions of the turbulent magnetic field and ion fluid velocity fluctuations, we find that the turbulence associated with electron-only reconnection shows the same statistical features as the turbulence associated with standard ion-coupled reconnection and no peculiar signature related to electron-only reconnection is found in the turbulence statistics. This result suggests that the properties of the turbulent cascade in a magnetized plasma are independent of the specific mechanism associated with magnetic reconnection but depend only on the coupling between the magnetic field and the different particle species present in the system. Finally, the properties of the magnetic field dissipation range are discussed as well and we claim that its formation, and thus the dissipation of magnetic energy, is driven only by the small scale electron dynamics since ions are demagnetized in this range [Arró et al., 2020].This work has received funding from the European Union Horizon 2020 research and innovation programme under grant agreement No 776262 (AIDA, www.aida-space.eu).References:G. Arró, F. Califano, and G. Lapenta. Statistical properties of turbulent fluctuations associated with electron-only magnetic reconnection. , 642:A45, Oct. 2020. doi: 10.1051/0004-6361/202038696.F. Califano, S. S. Cerri, M. Faganello, D. Laveder, M. Sisti, and M. W. Kunz. Electron-only magnetic reconnection in plasma turbulence. arXiv e-prints, art. arXiv:1810.03957, Oct. 2018.

In space and astrophysical plasmas magnetized coherent structures continuously emerge as an outcome of the nonlinear dynamics. These structures are characterized by the presence of localized strong current density peaks. Here we present a statistical study of the development of such structures resulting in different hybrid-Vlasov 3D-3V simulations of plasma turbulence. In particular, we make use of different methods to characterize the global shape of the 3D structures. Furthermore, we study the local magnetic configuration inside and outside current peak regions, comparing the statistics in the two cases. Finally, we discuss correlations between characteristic dimensions of the current structures and current density, marking the difference between magnetic structures in 2D and 3D simulations.

Context. In space and astrophysical plasmas, turbulence leads to the development of coherent structures characterized by a strong current density and important magnetic shears. Aims. Using hybrid-kinetic simulations of turbulence (3D with different energy injection scales), we investigate the development of these coherent structures and characterize their shape. Methods. First, we present different methods to estimate the overall shape of the 3D structure using local measurements, foreseeing an application on satellite data. Then we study the local magnetic configuration inside and outside current peak regions, comparing the statistics in the two cases. Last, we compare the statistical properties of the local configuration obtained in simulations with the ones obtained analyzing an MMS (Magnetospheric MultiScale mission) dataset having similar plasma parameters. Results. Thanks to our analysis, (1) we validate the possibility of studying the overall shape of 3D structures using local methods, (2) we provide an overview of a local magnetic configuration emerging in different turbulent regimes, (3) we show that our 3D-3V simulations can reproduce the structures that emerge in MMS data for the periods considered.

Kinetic turbulence in magnetized space plasmas has been extensively studied via in situ observations, numerical simulations, and theoretical models. In this context, a key point concerns the formation of coherent current structures and their disruption through magnetic reconnection. We present automatic techniques aimed at detecting reconnection events in a large data set of numerical simulations. We make use of clustering techniques known as K-means and DBscan (usually referred to in literature as unsupervised machine-learning approaches), and other methods based on thresholds of standard reconnection proxies. All our techniques also use a threshold on the aspect ratio of the regions selected. We test the performance of our algorithms. We propose an optimal aspect ratio to be used in the automated machine-learning algorithm: AR = 18. The performance of the unsupervised approach results in it being strongly competitive with respect to those of other methods based on thresholds of standard reconnection proxies.

International audience; In order to determine particle velocities and electric field in the frame of the magnetic structure, one first needs to determine the velocity of the magnetic structure in the frame of the spacecraft observations. Here, we demonstrate two methods to determine a two-dimensional magnetic structure velocity for the magnetic reconnection event observed in the magnetotail by the Magnetospheric Multiscale (MMS) spacecraft on July 11, 2017, Spatio-Temporal Difference (STD) and the recently developed polynomial reconstruction method. Both of these methods use the magnetic field measurements; the reconstruction technique also uses the current density measured by the particle instrument. We find rough agreement between the results of our methods and with other velocity determinations previously published. We also explain a number of features of STD and show that the polynomial reconstruction technique is most likely to be valid within a distance of 2 spacecraft spacings from the centroid of the MMS spacecraft. Both of these methods are susceptible to contamination by magnetometer calibration errors.

Density inhomogeneities are ubiquitous in space and astrophysical plasmas, in particular at contact boundaries between different media. They often correspond to regions that exhibits strong dynamics on a wide range of spatial and temporal scales. Indeed, density inhomogeneities are a source of free energy that can drive various instabilities such as, for instance, the lower-hybrid-drift instability which in turn transfers energy to the particles through wave-particle interactions and eventually heat the plasma. We aim at quantifying the efficiency of the lower-hybrid-drift instability to accelerate and/or heat electrons parallel to the ambient magnetic field. We combine two complementary methods: full-kinetic and quasilinear models. We report self-consistent evidence of electron acceleration driven by the development of the lower-hybrid-drift instability using 3D-3V full-kinetic numerical simulations. The efficiency of the observed acceleration cannot be explained by standard quasilinear theory. For this reason, we develop an extended quasilinear model able to quantitatively predict the interaction between lower-hybrid fluctuations and electrons on long time scales, now in agreement with full-kinetic simulations results. Finally, we apply this new, extended quasilinear model to a specific inhomogeneous space plasma boundary: the magnetopause of Mercury, and we discuss our quantitative predictions of electron acceleration in support to future BepiColombo observations., Comment: Paper under evaluation for publication on the journal "Astronomy and Astrophysics"

Context. In the last few decades, increasing evidence has been found in both numerical studies and high-resolution in situ data that magnetic turbulence spontaneously generates coherent structures over a broad range of scales. Those structures play a key role in energy conversion because they are sites where magnetic energy is locally dissipated in plasma heating and particle energization. How much turbulent energy is dissipated via processes such as magnetic reconnection of thin coherent structures, namely current sheets, remains an open question. Aims. We aim to develop semi-automated methods for detecting reconnection sites over multiple spatial scales. This is indeed pivotal in advancing our knowledge of plasma dissipation mechanisms and for future applications to space data. Methods. By means of hybrid–Vlasov–Maxwell 2D–3V simulations, we combine three methods based on the partial variance of increments measured at a broad range of spatial scales and on the current density, which together, and in a synergistic way, provide indications as to the presence of sites of magnetic reconnection. We adopt the virtual satellite method, which in upcoming works will allow us to easily extend this analysis to in situ time-series. Results. We show how combining standard threshold analysis to a 2D scalogram based on magnetic field increments represents an efficient diagnostic for recognizing reconnecting structure in 1D spatial- and time-series. This analysis can serve as input to automated machine-learning algorithms.

International audience

Context. Recent satellite measurements in the turbulent magnetosheath of Earth have given evidence of an unusual reconnection mechanism that is driven exclusively by electrons. This newly observed process was called electron-only reconnection, and its interplay with plasma turbulence is a matter of great debate. Aims. By using 2D-3V hybrid Vlasov–Maxwell simulations of freely decaying plasma turbulence, we study the role of electron-only reconnection in the development of plasma turbulence. In particular, we search for possible differences with respect to the turbulence associated with standard ion-coupled reconnection. Methods. We analyzed the structure functions of the turbulent magnetic field and ion fluid velocity fluctuations to characterize the structure and the intermittency properties of the turbulent energy cascade. Results. We find that the statistical properties of turbulent fluctuations associated with electron-only reconnection are consistent with those of turbulent fluctuations associated with standard ion-coupled reconnection, and no peculiar signature related to electron-only reconnection is found in the turbulence statistics. This result suggests that the turbulent energy cascade in a collisionless magnetized plasma does not depend on the specific mechanism associated with magnetic reconnection. The properties of the dissipation range are discussed as well, and we claim that only electrons contribute to the dissipation of magnetic field energy at sub-ion scales.

International audience; We investigate both large‐ and small‐scale properties of a Kelvin‐Helmholtz (KH) event at the dusk flank magnetopause using Magnetospheric Multiscale observations on 8 September 2015. We first use two types of 3‐D simulations (global and local) to demonstrate that Magnetospheric Multiscale is close to the most KH unstable region, and so the occurrence of vortex‐induced reconnection may be expected. Because they produce low‐shear current sheets, KH vortices constitute a perfect laboratory to investigate magnetic reconnection with large guide field and low asymmetry. Recent works suggest that magnetic reconnection may be suppressed when a current sheet combines large guide field and pressure gradient (which induces a diamagnetic drift). We thus perform a statistical analysis of high‐resolution data for the 69 KH‐induced low‐shear magnetic reconnection events observed on that day. We find that the suppression mechanism is not at work for most of the observed reconnecting current sheets, as predicted, but we also find that almost all nonreconnecting current sheets should be reconnecting according to this model. This confirms the fact that the model provides a necessary but not sufficient condition for reconnection to occur. Finally, based on the same data set, we study the latitudinal distribution of these magnetic reconnection events combined with global magnetospheric modeling. We find that reconnection associated with KH vortices occurs over a significant range of latitudes at the flank magnetopause. It is not confined to the plane where the growth rate is maximum, in agreement with recent 3‐D simulations.

We present an analysis of energy transfers in a reconnecting near-Earth plasma, obtained by interpreting MMS data within the framework of multi-fluid plasma theory. In our analysis, energy transfers are calculated and examined locally. This way, correlations between different mechanisms of energy exchange can be retrieved in all spatial and temporal detail provided by the high-frequency, multi-point sampling capacity of the four MMS satellites. In particular, compressional effects are separated from effective sources in the energy density evolution equations, allowing to distinguish whether some effective energy transfer is occurring locally. A large database of MMS encounters with reconnecting current sheets is exploited in order to assess the statistical validity of all results presented.

Kinetic turbulence in magnetized space plasmas has been extensively studied via in situ observations, numerical simulations and theoretical models. In this context, a key point concerns the formation of coherent current structures and their disruption through magnetic reconnection. As of today, reconnection can only be accurately identified by human analysis. We are setting-up a machine learning unsupervised technique aimed at automatically detecting the presence of current sheet (CS) magnetic structures where reconnection is occurring. We make use of anomaly detection and clustering techniques. We are applying these techniques to 2D kinetic HVM (Hybrid Vlasov Maxwell) plasma turbulence simulations, where ions evolve by solving the Vlasov equation and the electrons are treated as a fluid. Electron inertia is included. The final goal is to build up an algorithm able to select data subsets starting from big data sets where potentially interesting physical processes are at play. After that, we intend to extend the technique to space data and to 3D simulation data.This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 776262 (AIDA, www.aida-space.eu).

The understanding of fundamental processes at play in a collisionless plasmas such as the solar wind, is a frontier problem in space physics. We investigate here the occurrence of magnetic reconnection in a plasma with parameters corresponding to solar wind plasma and its interplay with a fully-developed turbulent state. Ongoing magnetic reconnection can, at the moment, be accurately identified only by humans. Therefore, as a first step, the goal of this study is to present a new method to automatically recognise reconnection events in the output of two-dimensional HVM (Hybrid Vlasov Maxwell) simulations where ions evolve by solving the Vlasov equation and the electrons are treated as a fluid with mass. A large dataset with labelled reconnection events was prepared, including parameters such as the magnetic field, the electron velocity field and the current density. We consider two types of machine learning models: classical approaches using on physics-based features, and convolutional neural networks (CNNs). We will investigate which approach performs better, and which input variables are most relevant. In addition, we will try to categorize magnetic reconnection regions (current sheets). This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 776262 (AIDA, www.aida-space.eu).

During periods of strong magnetic activity, cold dense plasma from the plasmasphere typically forms a plume extending towards the dayside magnetopause, eventually reaching it. In this work, we present a large-scale two-dimensional fully kinetic Particle-In-Cell simulation of a reconnecting magnetopause hit by a propagating plasmaspheric plume. The simulation is designed so that it undergoes four distinct phases: initial unsteady state, steady state prior to plume arrival at the magnetospause, plume interaction and steady state once the plume is well engulfed in the reconnection site. We show the evolution of the magnetopause's dynamics subjected to the modification of the inflowing plasma. Our main result is that the change in the plasma temperature (cold protons in the plume) have no effects on the magnetic reconnection rate, which on average depends only on the inflowing magnetic field and total ion density, before, during and after the impact., Comment: Letter, 13 pages, 2 figures, 1 table

Magnetic reconnection is a fundamental process that quickly releases magnetic energy stored in a plasma.Identifying, from simulation outputs, where reconnection is taking place is non-trivial and, in general, has to be performed by human experts. Hence, it would be valuable if such an identification process could be automated. Here, we demonstrate that a machine learning algorithm can help to identify reconnection in 2D simulations of collisionless plasma turbulence. Using a Hybrid Vlasov Maxwell (HVM) model, a data set containing over 2000 potential reconnection events was generated and subsequently labeled by human experts. We test and compare two machine learning approaches with different configurations on this data set. The best results are obtained with a convolutional neural network (CNN) combined with an 'image cropping' step that zooms in on potential reconnection sites. With this method, more than 70% of reconnection events can be identified correctly. The importance of different physical variables is evaluated by studying how they affect the accuracy of predictions. Finally, we also discuss various possible causes for wrong predictions from the proposed model., Comment: 16 pages, 9 figures and 5 tabels

Observation of the solar wind – magnetosphere boundary provides a unique opportunity to investigate the physics underlying the interaction between two collisionless magnetized plasmas with different temperature, density and magnetic field topology. Their mixing across the interface as well as the boundary dynamics are affected by the development of fluid (and kinetic) instabilities driven by large scale inhomogeneities in particle and electromagnetic fields. Building up a realistic initial equilibrium state of the magnetopause according to observations is still a challenge nowadays. In this paper we address the modeling of the particles and electromagnetic fields configuration across the Earth's magnetopause by means of a three-fluid analytic model. The model relies on one hot and one cold ion population and on a neutralizing electron population. The goal is to build up an analytic model able to reproduce as closely as possible the observations. Some parameters of the model are set by using a fit procedure aiming at minimizing their difference with respect to experimental data provided by the Magnetospheric MultiScale mission. All the other profiles, concerning the electron pressure and the relative densities of the cold and hot ion populations, are calculated in order to satisfy the fluid equilibrium equations. Finally, by means of a new tri-fluid code, we have checked the stability of the large-scale equilibrium model for a given experimental case and given the proof that the system is unstable to reconnection. This model could be of interest for the interpretation of satellite results and for the study of the dynamics at the boundary between the Magnetosphere and the solar wind.

International audience; Velocity shear driven vortices associated to the Kelvin-Helmholtz instability (KHI) have been detected by in-situ observations at the Earth, Saturn and Mercury magnetospheres' boundary due to the interaction with the solar wind. KHI in magnetized plasmas have been widely studied numerically in the framework of a fluid and hybrid descriptions, while only very few studies have focused on the physics of electrons because of computational constraints. In this work we present a full kinetic particle in cell study of the KHI spanning a range of scales going from fluid to electron scales. Within this framework, we discuss the formation of ion-scale magnetic holes caused by the development of an electron mirror instability in the region of the velocity shear. The observed magnetic holes are pressure balance structure, in which the deep in magnetic pressure is balanced by a bump in the kinetic pressure. The latter is mainly provided by an high electron temperature in the direction perpendicular to the guide field. A diamagnetic electron current ring flowing inside the magnetic holes is also observed. Our work suggests a possible mechanism for the generation of the ion-scale magnetic holes recently observed by MMS in the Earth's magnetosphere.

International audience; The Electron Diffusion Region (EDR) is the region where magnetic reconnection is initiated and electrons are energized. Because of experimental difficulties, the structure of the EDR is still poorly understood. A key question is whether the EDR has a homogeneous or patchy structure. Here we report Magnetospheric MultiScale (MMS) novel spacecraft observations providing evidence of inhomogeneous current densities and energy conversion over a few electron inertial lengths within an EDR at the terrestrial magnetopause, suggesting that the EDR can be rather structured. These inhomogenenities are revealed through multi-point measurements because the spacecraft separation is comparable to a few electron inertial lengths, allowing the entire MMS tetrahedron to be within the EDR most of the time. These observations are consistent with recent high-resolution and low-noise kinetic simulations.

著者人数: 39名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 長谷川, 洋; 齋藤, 義文), Number of authors: 39 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Hasegawa, Hiroshi; Saito, Yoshifumi), Accepted: 2019-06-10, 資料番号: SA1190161000

Inflammatory pseudotumor is a soft-tissue lesion of unknown etiology, but likely of reactive nature. It is considered benign and treated by simple excision; however, pre-operative diagnosis is often challenging because of the ability of pseudoinflammatory pseudotumor to mimic numerous benign and malignant entities, both clinically and ultrasonographically. Herein, we present the case of a 56-year-old man with a paratesticular inflammatory pseudotumor with concerning clinical features which could not be identified clinically or intra-operatively and thus required an orchiectomy to reach the final diagnosis. In the paratesticular region, inflammatory pseudotumors are uncommon (6% of masses) but must be kept in mind due to their benign nature.

In this paper, we investigate nonrelativistic, kinetic, linear phase of the filamentation instability when an external magnetic field is present in the direction of the counterstreaming electron beams using Vlasov simulations in 1D-3V space. We first investigate the growth rate of instability. In the linear growth regime, our results correspond to the previous conclusions that with the increase in strength of the ambient magnetic field, there is a suppression of instability. Interestingly, we established that at a critical / threshold magnetic field, Vlasov simulations and particle-in-cell (PIC) simulations differ in their instability behaviour. At this particular magnetic field, there is a complete suppression of the growth of instability in Vlasov results compared to PIC simulations, where a strong growth of instability is shown. It is believed that thermal noise in the PIC leads to the growth. However, Vlasov simulations show wave–wave coupling which stabilises the modes. In this work, our focus is to demonstrate the difference in this behaviour and to thoroughly analyse the spectra and wave generation for the same.

International audience; We present a 3-D two-fluid simulation using plasma parameters as measured by MMS on 8 September 2015 concerning the nonlinear development of the Kelvin-Helmholtz instability at the Earth's magnetopause. We observe an extremely rich nonlinear dynamics including the development of a complex magnetic topology, vortex merging, and secondary Kelvin-Helmholtz instability driven by large-scale vortices distributed asymmetrically in latitude. Vortex induced and midlatitude magnetic reconnection coexist and produce an asymmetric distribution of magnetic reconnection events. These results are in good agreement with MMS observations on the same day, in particular for the presence of both equatorial and off-equator reconnection. Regarding the latter only, we note a predominance of reconnection in the Southern Hemisphere during the early nonlinear phase. The estimated effective diffusion coefficient associated with the dynamics is found to be large enough to account for the observed mass transport at the Earth's magnetospheric flanks.

International audience; We propose a new numerical code based on a new multi-species theoretical model to study the mass, momentum and energy exchanges (MMEE) that happen across the magnetospheric boundaries. We use two distinct populations for ions, one cold and one hot (plus one neutralising electron population), to take into account the differences between the properties of the plasmas coming from the magnetosphere and from the solar wind. This approach represents a step forward in the context of the study of coupled large-scale plasma systems being a new and efficient compromise between fluid and kinetic codes in tracing the different plasma contributions during MMEE. Due to the very important role that magnetic reconnection plays in connecting the shocked Solar Wind to the Earth's magnetosphere, we show and discuss the results we obtained about the simulations of the tearing mode instability occurring across an Earth's magnetopause that we modelled thanks to our most recents MMS observations [Rezeau 2018]. &